Electric heater



Patented June 23, 1953 ELECTRIC HEATER William R. Tuttle, Riverside, Ill., assignor to Tuttle & Kift, Inc., Chicago, Ill., a corporation of Illinois Application February 11, 1950, Serial No. 143,631

Claims.

My invention relates to electric heaters of the type wherein a resistance is embedded in granular or powdered refractory insulating material which is compacted in a hard dense mass within a tubular metal sheath, and has reference more particularly to the cross sectional shape thereof. This application is a continuation in part of my application Serial No. 611,474 (Patent 2,508,552), filed August 20, 1945, which is a continuation in part of my applications Serial Nos. 566,855 (Patent 2,456,343) and 566,856 (Patent 2,506,554), both filed on December 6, 1944.

Such heaters are generally made in a nonrectilinear form depending upon the purpose for which the heater is intended to be used, a common form employed for surface heating units of electric ranges being a flat spiral wherein the tube is shaped in cross section to provide a wide flat surface throughout the active length thereof at one side of the spiral, to increase the heating surface presented to a utensil placed on the heating unit.

The principal object of my invention is to provide a new and improved tubular sheathed embedded-resistor electric heater of the type in which the tube is flattened at one side.

In the accompanying drawing:

Fig. 1 is a top plan view of an electric heater embodying my invention, said heater being shown for purpose of illustration, in a fiat spiral form suitable for use as a surface heater for electric ranges;

Fig. 2 is an enlarged fragmentary sectional view of the outer end portion of the spiral heater taken substantially on the line 2-2 of Fig. l;

Fig. 3 is an enlarged fragmentary sectional view of the inner end of the spiral heater taken substantially on the line 33 of Fig. 1;

Fig. 4 is a sectional view substantially on the line 4-4 of Fig. 3;

Fig. 5 is a cross sectional view of the heater interposed between shaping dies which are closed sulating material ll within which is embedded the resistance or resistances which constitute the electric heating element, two resistances l2 and I3 and a return conductor M (see Fig. 1) being shown herein for purposes of illustration. The tube I0 preferably has a circumferentially continuous wall and together with the resistances and insulation therein may be bent in spiral form, such as shown at [5 in Fig. 1, or may be in other desired form, the spiral form being shown for purposes of illustration as it is particularly suitable for surface heating units of electric ranges in which my present invention is particularly advantageous.

Any suitable insulation ll may be employed, as for example, finely divided fused magnesium oxide which is compacted in the tube Ill to a hard dense mass so as to have high heat conductivity and hold the resistances I2 and I3 and return conductor l4 securely in place in proper insulated relation to one another and to the metal tube or sheath [0.

The tube or sheath In at the inner end of the spiral i5 is hermetically sealed at l6 and within the sealed end the inner end of the return conductor I4 is joined to the inner ends of the resistances l2 and I3 in any convenient manner, a form of connection that may be employed being disclosed in my aforesaid application Serial No. 566,856.

The resistances I2 and 13 are elongated windings or coils of resistance wire each initially in the form of a cylindrical helix and subsequently deformed transversely as hereinafter explained, and the return conductor I 4 instead of being a single wire which is connected at the closed end IE to both resistances l2 and Has explained above, may be a pair of wires l2a and [3a closely adjoining or twisted together, the wire 120, being an integral extension of the wire of the resistance l2 and stretched out straight or nearly straight and bent back at i2b as shown in Figs. 3 and 4, and the wire [311. being a similar integral extension of the wire of the resistance !3 and likewise bent back as at [31). These wires l2a and [3a are connected to a single terminal at the outer end of the spiral heating unit [5.

In the particular electric heater disclosed here in, the spiral l5 terminates at its outer end in a tangential portion I! which is bent downwardly at an angle to the plane of the spiral l5, substantially as shown in Fig. 2, and this portion I! has an extension tube [8 secured thereto and into this extension tube extend three separate insulated-conductors or leads I9, 20 and 2| which supply current to the resistances l2 and i3, said leads ii 252 being connected respectively to the outer ends of the resistances i2 and I3 and the lead all being connected to the outer end of the return conductor l4, all as disclosed in ny aforesaid application Serial No. 6l1,i74=.

The spiral tube [5 is flattened at the top throughout the length thereof, as indicated at 22, from the down-turned end IT to the inner closed end iii to provide a wide surface area for supporting and heating a utensil thereon and the opposite sides 23 of the tube below this flattened top 22 converge downwardly in a general V shape form so that the general cross sectional shape of the flattened length of the spiral i5 is triangular.

It is an important feature of thepresent in" vention, however, that these opposite sides 23 of the tube H3 below the flattened top 22 do not converge in a form closely approximating a V- shape, but instead each of these opposite sides 23 is curved inwardly as shown particularly in Fig. 4 so that the outer side of each side wall 23 is of concave form and the inner side of each side wall 23 of convex form from the downturned outer end portion ll of the spiral to the closed inner end l6 thereof.

The lateral edge portions 24 are rounded as shown in Fig. i where the sides 23 join the top 22 and there is also a rounded edge 25 along the bottom of the tube l where the inwardly curved sides 23 join one another, each side wall 23 throughout its length being of inwardly curved channel or trough shape cross sectional form between the rounded bottom 25 and the respective rounded lateral edge 24.

In making this heater, a straight metal tube of cylindrical form and suitable length to produce the eventual spiral I is employed for producing the sheath I 9. This cylindrical tube, which is indicated at [to in Fig. 7, is preferably provided with an index notch 26 (see Fig. '7) at one or both ends for a purpose hereinafter explained.

Then cylindrical helices, which are indicated at I and 13c in Fig. '7 and which eventually become the resistances l2 and I3 respectively, together with their return wires l2a and I311v (or return wire [4) are inserted in the tube lilo and held at the opposite ends of the tube in aligning matched positions, as determined by the aforesaid index notch or notches 21, and in proper spaced relation from one another and from the tube and outstretched under sufficient tension so that they extend throughout the tube in proper spaced apart relation with one another and with the tube, and in exact parallelism with the axis of the tube. The helices I20 and I3c are substantially equally spaced circumferentially from one another and from the return wire or wires and when thus installed are located at one side of the tube and the return wire or wires at the opposite side, the particular side of the tube at which said helices I20 and [3c are located being selected and determinable thereafter in the further processing by the aforesaid index notch or notches 2i of the tube.

Then the tube with said helices and return wire or wires therein is held in upright position, and gradually filled with the powdered or granular insulating material which is compacted, during the filling, by conventional tamping or by sidetapping, or upright jolting or both, so that at the completion of the filling the insulation is tightly compacted and retains the resistances and return wire or wires securely in place in the tube. It is to be understood, of course, that the tapping and jolting of the tube must be insufficiently severe to displace the resistances and return wire or wires from the positions in which they were installed.

Thereafter it is desirable to further compact the insulation by sidewise compression of the straight tube and its contents, and this may be accomplished by interposing and compressing the assembly sidewise between suitable opposed flattening dies of suitable stroke and pressure to impart the desired hardness to the compacted insulation, it being essential, of course, to have the ends of the tubes closed at the time sufiiciently securely to prevent extrusion of the insulation therefrom during the compacting.

Removable fittings may be provided for closing the two ends of the tube during compacting, but with the disclosed heater I prefer to employ a removable fitting at only one end and permanently close the other end hermetically.

Such permanent closure may be accomplished by inserting a substantially hemispherical metal cap 28 into the tube end which is to be permanently closed and forcing it tightly against the insulation so that the interior of the cap is completely filled therewith, and then welding the surrounding margin of the tube Hlc to the cap 28 entirely around the latter as indicated at 28c. This closed end of the tube [00 eventually becomes the closed inner end [6 of the spiral [5.

Before applying the cap 28, additional insulation is applied in that end of the tube l0c if necessary and tamped compactly to completely embed the outer ends of the helices [2c and I30 and of the return wire I4 or wires l2a and Hat and the end connections therebetween and to fill that end of the tube to the location where the cap 28 is to be secured thereon, and thereby insure insulation of the resistances and return wires from the cap 28 throughout the completion of the heater.

The other end of the tube I00 is closed by a removable fitting 29 which comprises a cylindrical plug portion 30 which is inserted in the tube [0c and a disk like head 3| and at the juncture of the head 3| and plug 30 the fitting 29 is provided with a lug 32 which fits into the tube notch 21 and keys the fitting 29 to the tube 10c in one certain position. For a purpose hereinafter explained, the head 3| of the fitting 29 is provided at intervals therearound with short radially projecting pins 33.

The fitting 29 closes that end of the tube I00 which subsequently becomes the tangential outer end I! of the spiral l5 through which the leads or conductors l9, 2!) and 2| are connected respectively to the resistances l2, l3 and return wire M or return wires [2a and Na and this fitting 29 has three openings therethrough in alignment respectively with the coils 12c and and return wire Hi or combined wires I2a, and Na to accommodate terminals 34, 35 and 36 which are connected respectively with the outer ends of the helices 12c and 130 and return wire M or combined return wires l 2a and l3a.

Such connection may be made by providing each of the terminals 34 and 35 with a threaded inner end which screws tightly in the spiral outer end of the respective helices 12c and [30, as indicated at 37, and if return wires |2a and i3a are employed the outer ends thereof are conjointly formed spirally to receive the threaded inner end of a terminal 36 in a manner similar to the; connection 31 of the terminals 34 and 35.

If, however, a single return wire (4 is employed, the outer end thereof may itself project through the proper opening in the fitting 29 to provide the terminal 36.

These terminals 34, 35 and 36 have the fittin 29 secured thereto, as for example by set screws 38, to hold the fitting in place in the end of the tube lilc'during the making of the heater, but this fitting is eventually removed and the terminals cut off to prope length (and a portion of the tube lllc also at that end if necessary) and connected to the respective leads or conductors I9, 20 and 2! within the end of the heater in a manner such as indicated at 39 in Fig. 2.

The straight length of tube I and contents as thus prepared with the one end closed as at 28 and with the fitting 29 in the other end, is laid between opposed dies and compressed, after which the tubewith its compressed contents is shaped in the spiral form, shown at IS, with the closed end atthe inner end of the spiral as indicated at 1'6, and with the other end, which was closed during processing by the removable fitting, at the periphery of the spiral and projectin tangentially therefrom as shown in Fig. 1.

In this compressing of the straight tube and contents, only the portion of the tube beyond the fitting 29 is compressed and at such distance from the fitting 29 that the latter is not affected by the compression.

Thereafter the spirally formed tube and contents are again compressed between dies to flatten the top of the tube throughout its active length, one of which dies is a flat die as indicated at 40 in Figs. and 6, and imparts the flat face 22 to the tube assembly and the other of which said dies, indicated at 4|, has a special groove 42 therein corresponding to the spiral form of the heater and of a cross sectional shape to impart the desired generally triangular cross sectional shape to the heater with concave sides 23.

This flattening of the top of the spirally formed heater provides a large area of contact with a utensil placed thereon and closer proximity thereto of the heating resistances l2 and [3, which in the flattening operation are deformed cross sectionally into a somewhat oval shape, somewhat as shown in Figs. 4 and 5, while the concave sides 23 impart greater rigidity to the heater and also serve topush up the under sides of the resistances I2 and I3 closer to the top of the unit where the heat therefrom is intended to be utilized.

For purposes of the present invention I prefer to compress the straight tube and contents into a generally square cross sectional form such as shown at la in Fig. 6 before the spiral winding and compression thereof into the final cross sectional form, shown in Figs. 4 and 5, as this facilitates the location of the resistances l2 and I3 in the correct final location and the shaping thereof in the desired final form.

Accordingly, after the tube I00 has been filled and closed at one end by the cap 28,,and while the fitting 29 is still located in place in the outer end of the tube, all as indicated in Fig. '7, I subject the straight tube and contents to a two way sidewise flattening operation which reduces the tube to the generally square cross sectional form shown at 42 in Fig. 6.

This two way sidewise flattening operation may be accomplished by first interposing and compressing the tube and contents between parallel pressure surfaces of opposed fiat dies which flatten the opposite sides of the tube, and thereafter turning the tube and contents circumferentially a quarter turn distance and subjecting the tube and contents again to the compressing operation of the same flattening dies, it being understood that the entire fittin 29 and a short length of the tube and contents therebeyond are located beyond the flattening dies during these flattening operations to avoid any deformation of the fitting 29.

it It is very important to maintain the helices I20 and 13a throughout all operations, at that side of the tube which will eventually become the top of the finished heater l5, and accordingly in the flattening of the tube I00 and contents to the square form Illa, it is essential that the tube and contents must be positioned circumferentially between the flattening dies so that both helices [2c and 130 will lie similarly close to, and with their longitudina1 axis in a plane exactly parallel with, a selected flat side of the tube, when the tube and contents are compressed to the square form 42.

v This is accomplished by arranging the indexin notch 21 at a precise circumferential position relatively to the two helices I20 and I30 installed therein, for example midway therebetween as indicated in Fig. '7, and locating the indexing lug 21 of the fitting 29 in an exact circumferentia1 position relating to the pins 33 of the fitting 29, for example in the plane of two opposed pins 33 (which latter pins are in a plane at right angles to the plane of the other two opposed pins 33), and employing the pins 33 to locate the tube I00 and contents in the particular circumferential positions in which it is flattened to the square form 42.

For this purpose, the flat die on which the tube and contents is placed in the squaring operations, has a pair of laterally spaced indexing blocks, such as indicated by dotted lines at 43 in Fig. 7, spaced marginally from the pressure face area of the dies, to engage the pins 33 in a manner to locate the tube and contents in the correct circumferential position between the dies and also in such longitudinal position that the fitting 29 is the proper distance beyond the pressure area of said dies.

The lower pressure die, which is indicated by dotted lines at 44 in Fig. 7, may have an extended ledge 45 on which the indexing blocks 43 are mounted in such spaced apart relation to admit the disk shaped head 3! therebetween, and each block 43 has a corresponding notch 46 in the top to receive a pin 33 therein, this arrangement being such that the positioning of either opposed pair of pins 33 in the notches 46 not only holds the tube and contents in the proper lengthwise position for flattening between the die 44 and its companion die thereabove, but also positions the tube and contents in such circumferential position that the helices [2c and are similarly close to and in a plane exactly parallel with the flattened side of the resultant fiat sided square tube 10a, which said flattened side is indicated at 41 in Fig. 6, and eventually becomes the top wall 22 of the finished heater.

For convenience the flattened side of the square tubeopposite said flattened side 4'! is indicated by the reference numeral 48, and the two other opposed sides of the square tube are indicated by the reference numeral 49.

This compression of the tube I00 and its contents into the square form [9a is advantageous as it not only compacts and solidifies the insulation so as to effectivelyshape the helices I20 and I3c in the subsequent desired distorted cross sectional form, but it also facilitates winding of the tube and contents in the spiral form I without substantial twisting of the tube which might occur, if the tube were round, and result in some deviation of the helices, in their final form I2 and I3, from exact proper position they are to occupy in the top of the tube in its final form.

This square compression of the tube and contents is particularly advantageous, however, in the respect that it provides a flat side 41 for engagement by the upper die 4!! in the initial stage of final shaping so that the initial pressure of the dies thereon orients the tube, throughout its length, in flat face to face engagement with the upper die at], thereby correcting any twist that may have occurred in winding and insuring the positioning and maintaining of the two helices in a plane parallel to the upper die II] throughout the compression and shaping into the final generally triangular form I0, and is also particularly advantageous in the further respect that the compression to substantially square cross sectional form imparts a preliminary cross sectional shaping to the helices which contributes to the shape thereof in the final cross sectional form.

In this compression of the tube I00 and its contents into the substantially square form Illa of Fig. 6, the pressure that is applied to the four sides of the tube deforms the tightly compacted insulation therein and causes it to flow toward the portions of the tube that are becoming the rounded corners of the tube, and this cornerwise flow of the compacted insulation distorts the helices I and I cross sectionally and distends each transversely toward the nearest corner into a somewhat egg-shaped form, as shown at I202 and I3cl in Fig. 6, with the smaller end of the eggshaped form extending into the nearest corner that is formed by the flattening of the tube to the generally square form.

Thus the helices I20 and I30 of Fig. '7 are distorted into the cross sectional shape I2cl and l3d of Fig. 6 which contributes to the eventual fiattoning of these helices and widening of the side thereof next to the top wall 22 as indicated at 12 and I3, which is accomplished by the dies and ll and concentrates the heat from the resistances I2 and I3 close to the top face 22 of the heater and insures high efficiency and quick heating of a utensil placed thereon.

In the final shaping of the heater between the dies ll] and ll, the lower corners of the squared tube Ito are pushed in by the convex side wall portions of the die M and the middle portion of the bottom wall 48 of the tube is forced downwardly into the rounded bottom 5| of the die 4| while the upper portions of the tube side walls 49 are forced outwardly into the lateral offsets 52 along the opposite sides of the die groove 42 at the top so as to greatly increase the width of the top face 22 of the tube II) as indicated in Figs. 4 and 5.

Thus in the compressing of the spiral tube and contents between the dies 40 and 4| the tube wall is greatly distended across the top while at the same time the tube wall is bulged upwardly and inwardly at each side by the convex sides 50 of the lower die, which said bulging causes the insulation to flow upwardly-from the lower corners of the tube and outwardly toward each side of the tube at the top, and as the partially deformed helices I2d and [3d are located at the top of the tube, this insulation flow flattens the helices [2d and I3cZ in a direction toward the top of the heater and distends each outwardly toward therespective side of the heater into the some-what elongated pear shape cross sectional form shown at I2 and I3 in Figs. 4 and 5, with the smaller ends of the pear shape in the side folds of the tube wall along each side of the top portion thereof.

The top portions of these cross sectionally pear shaped coils or resistances I2 and I3 are generally fiat and very. close to and generally parallel with the top wall 22, whereas the bottom portions of the coils are shaped more nearly like the side of a pear, but each coil I2 and I3 is flattened sidewise toward the top wall 22 to such extent and distended laterally under the wall 22 to such width that the heat therefrom is concentrated in a particularly wide zone immediately underneath the top wall 22 thereby greatly increasing the rapidity of heattransfer to a utensil placed on the heater and providing an improved electric heater of greatly increased efficiency.

Thisfiattening and widening of the coils or resistances l2 and E8 is effected to some extent, as explained above, by the initial SideWise elongation of the coils by the compression of the tube into the generally square form Mic but is occasioned principally by the concavity imparted, throughout their length, to the side walls 23 of the tube, which are arranged in a generally V-sl1aped relation to one another but with considerable concavity and closed together at the bottom in a rounded contour as indicated at 25.

It is important, in heaters of the character above described that the tube or sheath Ill be of such. rigidity in its final form that it will not only hold the insulation II therein tightly compacted condition, but will effectively resist Warping of the heater, especially spiral forms thereof, in the temperature changes to which the heater is subjected.

My present cross sectional form of heater has the additional advantage that the concavity of the sides 23 imparts such rigidity thereto that the insulation is held more tightly compacted and, furthermore, 1 the concavity of the sides 23 throughout the active length of the heater effectively reinforces the heater against warping and twisting strains, which are particularly undesirable in surface heaters of spiral or similar types as they distort the top surface of the heater and thereby reduce the area of contact of the heater with a utensil placed thereon and appreciably diminish the conductivity of heat thereto.

While I have shown and described my invention in a preferred form, I am aware that various changes and modifications may be made therein without departing from the principles of my invention, the scope of which is to be determined by the appended claims.

What is claimed is:

1. An electric heater comprising a tubular metal sheath filled with a compacted mass of granular refractory electrical insulation which conforms to the interior surface of the sheath and has an electrical resistance coil embedded therein and extending lengthwise of the sheath and separated electrically therefrom by said insulation, said sheath having a convex wall portion extending along one side thereof and a relatively wide fiat wall portion extending along the opposite side thereof, said sheath having a convex wall portion along each lateral margin of said flat wall portion, and a pair of divergent reentrant concave wall portions extending respectively between each of the last mentioned convex wall portions and the first mentioned convex wall portion, each said concave wall portion being bent inwardly against a corresponding concave face of the mass of granular insulating material and holding said insulation compressed between said concave wall portion and the aforesaid flat wall portion and against the resistor.

2. An electric heater comprising a tubular metal sheath filled with a compacted mass of granular refractory electrical insulation which conforms to the interior surface of the sheath and has an electrical resistance coil embedded therein and extending lengthwise of the sheath and separated electrically therefrom by said insulation, said sheath having a convex wall portion extending along one side thereof and a relatively wide flat wall portion extending alongthe opposite side thereof, said sheath having a convex wall portion along each lateral margin of said flat wall portion, and a pair of divergent reentrant concave wall portions extending respectively between each of the last mentioned convex wall portions and the first mentioned convex wall portion, each said concave Wall portion being bent inwardly against a corresponding concave face of the mass of granular insulating material and holding said insulation compressed between said concave wall portion and the aforesaid flat wall portion and against the resistor, the radius of curvature of said first mentioned convex wall portion being substantially greater than the radius of curvature of the convex wall portions along the lateral margins of the flat wall portion.

3. An' electric heater comprising a tubular metal sheath containing a compacted mass of granular refractory electrical insulation and an electrical resistance coil embedded in said insulation and extending lengthwise of the sheath and separated electrically therefrom by said insulation, said sheath having a convex wall portion extending along one side thereof and a relatively wide flat wall portion extending along the opposite side thereof and having a convex wall portion along each lateral margin thereof, and a pair of divergent reentrant concave wall portions extending respectively between each of the last mentioned convex wallportions and the first mentioned convex wall portion and diverging from the latter wall portion to said convex wall portions along the lateral margins of the fiat wall portion, said electrical resistance coil being located substantially entirely between said fiat wall portion and a plane midway between said flat wall portion and the'opposite convex side of the sheath.

4. An electric heater comprising a tubular metal sheath containing a compacted mass of granular refractory electrical insulation and an electrical resistance coil embedded in said insulation and extending lengthwise of the sheath and separated electrically therefrom by said insulation, said sheath having a convex wall portion extending along one side thereof and a relatively wide flat wall portion extending along the opposite side thereof and having a convex'wall portion along each lateral margin thereof, and a pair of divergent reentrant concave wall portions extending respectively between each of the last mentioned convex wall portions and the first mentioned convex wall portion and diverging from the latter wall portion to said convex wall portions along the lateral margins of the flat wall portion, said sheath having a metal closure at one end which hermetically seals that end and is of a shape conforming to the aforesaid flat wall and opposite convex wall and reentrant wall portions of the sheath, said electrical resistance coilbeing located substantially entirely between saidflat wall portion and a plane midway between said flat wall portion and the opposite convexside of the sheath and having a return conductor leading therefrom at the closed end of the sheath to the other end thereof and located between said plane and said opposite convex side of the sheath.

5. An electric heater comprising a tubular metal sheath containing a compacted mass of granular refractory electrical insulation and an electrical resistance coil embedded in said insulation and extending lengthwise of the sheath and separated electrically therefrom by said insulation, said sheath having a convex wall portion extending along one side thereof and a relatively wide fiat wall portion extending along the opposite side thereof and having a convex wall portion along each lateral margin thereof, and a pair of divergent reentrant concave wall portions extending respectively between each of the last mentioned convex wall portions and the first mentioned convex wall portion and diverging from the latter wall portion to said convex wall portions along the lateral margins of the flat wall portion, said electrical resistance coil being located substantially entirely between said flat wall portion and a plane midway between said flat wall portion and the opposite convex side of the sheath, and flattened in a direction toward said flat wall portion to a cross sectional shape of substantially greater extent in a direction parallel with said fiat wall portion than in a direction perpendicular thereto.

6. An electric heater comprising a tubular metal sheath containing a compacted mass of granular refractory electrical insulation and two electrical resistance coils embedded in said insulation and extending lengthwise of the sheath in spaced apart parallel relation and separated electrically from the sheath and from one another by said insulation, said sheath having a convex wall portion extending along one side thereof and a relatively-wide fiat wall portion extending along the opposite side thereof, and having a convex wall portion along each lateral margin thereof, and a pair of divergent reentrant concave wall portions extending respectively between each of the last mentioned convex wall portions and the first mentioned convex wall portion and diverging from the latter wall portion to said convex wall portions along the lateral margins of the flat wall portion, both of said resistance coils being located substantially entirely between said fiat wall portion and a plane midway between said fiat wall portion and the opposite convex side of the sheath and both lying in a plane parallel with said flat wall portion, and each coil being flattened in a direction toward said flat wall portion to a cross sectional shape of substantially greater extent in a direction parallel with said fiat wall portion than in a direction perpendicular thereto.

7. The method of making an electric heater having a tubular sheath containing a compacted mass of granular refractory electrical insulation with an electrical resistance coil embedded there in, which said method includes the steps of first arranging the resistance coil lengthwise in and eccentrically at one side of a tube containing insulation compacted sufficiently to hold the resistance in place eccentrically in the tube, then compressing the tube sidewise into a generally square cross sectional form. with the resistance coil substantially closer to one flat side thereof than to the opposite flat side thereof and simultaneously redistributing and further compacting the insulation therein, then winding the squared tube in a flat spiral form with the two mentioned fiat sides of the squared tube at opposite sides respectively of the flat spiral, and thereafter compressing the spirally formed tube into generally triangular cross sectional form and widening that flat side thereof to which said resistance coil is substantially closer as aforesaid, and simultaneously shaping the aforesaid opposite flat side of the sheath to external convex form and simultaneously shaping the other two sides of the sheath to reentrant external convex form.

8. The method of making an electric heater having a tubular sheath containing a compacted mass of granular refractory electrical insulation with an electrical resistance embedded therein, which said method includes the steps of first arranging the resistance lengthwise in a tube containing insulation compacted sufficiently to hold the resistance in place in the tube, then compressing at least a portion of the length of the tube sidewise into a generally square cross sectional form and simultaneously redistributing and further compacting the insulation therein, then winding the squared portion of the length of the tubing in a flat spiral form with opposed flat faces of the squared tube at the opposite sides respectively of the flat spiral, and thereafter pressing those corner portions of the squared tubing which are along the lateral margins of one of said opposed flat faces inwardly toward one another and toward the other of said opposed fiat faces and simultaneously widening the latter face, and throughout the aforesaid procedure maintaining said resistance closer to that side of the sheath at which the last mentioned face is located than to the opposite side of the sheath.

9. The method of making an electric heater having a tubular sheath containing a compacted mass of granular refractory electrical insulation with an electrical resistance embedded therein, which said method includes the steps of first arranging the resistance lengthwise in a tube containing insulation compacted sufiiciently to hold the resistance in place in the tube, then compressing at least a portion of the length of the tube sidewise into a generally square cross sectional form and simultaneously redistributing and further compacting the insulation therein, then winding the squared portion of the length of the tubing in a flat spiral form with opposed flat faces of the squared tube at the opposite sides respectively of the flat spiral, and thereafter pressing those corner portions of the squared tubing along the lateral-margins of one of said opposed flat faces inwardly toward one another and toward the other of said opposed flat faces to a reversely bent shape and simultaneously widening the latter face, and throughout the aforesaid procedure maintaining said resistance closer to that side of the sheath at which the last mentioned face is located than to the oppo-v site side of the sheath.

10. The method of making an electric heater having a tubular sheath containing a compacted mass of granular refractory electrical insulation with two electrical resistances embedded therein, which said method includes the steps of first arranging the two resistances lengthwise in a tube containing insulation compacted sufficiently to hold the resistances in place in the tube, then compressing at least a portion of the length of the tube sidewise into a generally square cross sectional form with the two resistances laterally spaced from one another in a plane at one side of the longitudinal axis of the tube and parallel with two opposed fiat faces of the tube and simultaneously redistributing and further compactin the insulation therein, then winding the squared portion of the length of the tubing in a flat spiral with said two opposed flat faces of the squared tube at opposite sides respectively of the flat spiral, and thereafter pressing the corner portions of the squared tubing along the lateral margins of the one of said opposed flat faces which is farthest from said resistances inwardly toward one another and toward the other of said opposed fiat faces to a reversely bent shape and simultaneously widening the latter face, and throughout the aforesaid procedure maintaining both of said resistances closer to that side of the sheath at which the last mentioned face is located than to the opposite side of the sheath.

WILLIAM R. TUTTLE References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 749,240 Conill Sept. 29, 1903 2,322,988 Wiegand June 29, 1943 2,456,343 Tuttle Dec. 14, 1948 2,462,016 W'iegand Feb. 15, 1949 FOREIGN PATENTS Number Country Date 528,718 Great Britain c Nov. 5, 1940 

